1. Field of the Invention
The present invention relates to a projector (projection display apparatus) that projects and displays images.
2. Description of the Related Art
The projector generally uses light emitted from a light source unit as illumination light to irradiate an illumination area (light illumination plane) of an electro-optic device. The electro-optic device modulates the incident light entering through its light illumination plane according to image information (image signals), and outputs image light representing a resulting image. The image light output from the electro-optic device is projected on a screen via a projection optical system, so that a resulting image is displayed. Typical examples of the electro-optic device include micro-mirror light modulators, such as a Digital Micro-mirror Device (DMD, trade mark by TI Corp.) and liquid crystal panels. The DMD is an electro-optic device having a higher utilization efficiency of the light irradiating the light illumination plane, compared with the liquid crystal panel.
A projector utilizing one electro-optic device (called ‘single-panel projector’) may attain color display in the following manner. The single-panel projector uses a color wheel including three color filters corresponding to three primary colors, red (R), green (G), and blue (B). The color wheel cyclically and sequentially allows transmission of the three primary color light components, R, G, and B included in the incident illumination light. The three primary color light components transmitted through and output from the color wheel sequentially irradiate the light illumination plane of the electro-optic device. The electro-optic device modulates the sequentially irradiated color light components according to corresponding color signals and thereby generates image rays corresponding to the color signals. Images corresponding to the generated image rays of the respective colors (called ‘color component images’) are sequentially projected. The persistence of vision of human eyes combines the sequentially projected three color component images to one composite color image.
Color light components that are not transmitted through the color wheel are wasted in the process of sequentially displaying the color component images corresponding to the three primary color components, R, G, and B and generating a resulting color image. This technique is called time sharing display or field sequential display. When the color wheel transmits the R color light component, the G and B color light components are wasted. When the color wheel transmits the G color light component, the B and R color light components are wasted. When the color wheel transmits the B color light component, the R and G color light components are wasted. This arrangement accordingly does not allow the light emitted from the light source unit to be fully utilized as illumination light, and lowers the light utilization efficiency.
A method of solving the drawback discussed above and enhancing the light utilization efficiency has been proposed in the reference ‘Sequential Color Recapture and Dynamic Filtering: A Method of Scrolling Color’ (D. Scott Dewald, Steven M. Penn, and Michael Davis, Society for Information Display 2001. International Symposium Digest of Technical Papers, Volume XXXII, page 1076.
FIG. 5 is a plan view schematically illustrating the main part of a prior art projector 1000 based on SCR (Sequential Color Recapture) technique, which is described in the above cited reference. This projector (hereafter referred to as ‘SCR projector’) 1000 includes a light source unit 100, an integrator rod for SCR (hereafter referred to as ‘SCR integrator’) 200, a color wheel for SCR (hereafter referred to as ‘SCR wheel’) 300, a relay optical system 400, a reflecting mirror 500, a field lens 600, a DMD 700, and a projection lens (projection optical system) 800, which are sequentially arranged along a system optical path (central axis) 1000ax. 
The light emitted from the light source unit 100 is condensed light, which is substantially focused on a light entrance plane 202 of the SCR integrator 200 on the system optical axis 1000ax and thereby efficiently enters through the light entrance plane 202. More specifically, the light source unit 110 includes an elliptical reflector 110 having a spheroidal reflection plane and a light source lamp 120, which is a high-pressure discharge lamp, such as a metal halide lamp or a high-pressure mercury lamp. The light source lamp 120 is located at a first focal point F1 of the elliptical reflector 110, and the SCR integrator 200 is arranged to locate its light entrance plane 202 at a second focal point F2 of the elliptical reflector 110. The light source unit 100 of this construction emits the condensed light, which is substantially focused on the light entrance plane 202 of the SCR integrator 200 on the system optical axis 1000ax. 
An image focused on the light entrance plane 202 of the SCR integrator 200 (hereafter referred to as ‘secondary light source image’) has a dimension (either a dimension in a lateral direction or a dimension in a vertical direction) DSP, which is expressed by Equation (1) given below:DSP=DA·f2/f1  (1)where DA denotes a dimension (either a dimension in the lateral direction or a dimension in the vertical direction) of an arc image of the light source lamp 120, f1 denotes a first focal length of the elliptical reflector 110, and f2 denotes a second focal length of the elliptical reflector 110.
As clearly understood from Equation (1), the dimension of the secondary light source image is adjustable by selecting adequate one among multiple elliptical reflectors having different first focal lengths f1 or different second focal lengths f2.
The light entering the SCR integrator 200 through the light entrance plane 202 is repeatedly reflected inside the SCR integrator 200 and is output from a light outgoing plane 204. The SCR integrator 200 makes the incident light repeatedly reflected therein and thus functions to convert the incident light of a non-uniform illuminance distribution entering through the light entrance plane 202 into light of a uniform illuminance distribution and output the converted light of the uniform illuminance distribution from the light outgoing plane 204.
FIG. 6 shows the structure of the SCR integrator 200. FIG. 6(B) is a plan view of the SCR integrator 200. FIG. 6(A) is a side view of the SCR integrator 200 on the side of the light entrance plane 202. FIG. 6(C) is a side view of the SCR integrator 200 on the side of the light outgoing plane 204. The SCR integrator 200 is a translucent rod of a quadratic prism having rectangular light entrance plane 202 and light outgoing plane 204. The light outgoing plane 204 generally has the contour similar to the contour of a light illumination plane (illumination area) of the DMD 700 by taking into account the illumination efficiency on the light illumination plane. For example, the light illumination plane of the DMD 700 has an aspect ratio (horizontal to vertical ratio) of about 4 to 3 or about 16 to 9. The light outgoing plane 204 is thus designed to have the similar aspect ratio of about 4 to 3 or about 16 to 9.
A reflecting mirror 206 is formed on the surface of the light entrance plane 202 to have a reflection plane in contact with the light entrance plane 202. The reflecting mirror 206 has a circular opening 206a around a central axis 200ax of the SCR integrator 200, which is located to be coincident with the system optical axis 1000ax. Only the light passing through the opening 206a can enter the SCR integrator 200 through the light entrance plane 202. It is accordingly preferable to adjust the dimension DSP of the secondary light source image expressed by Equation (1) to be smaller than the dimension of the opening 206a. This arrangement enables the light to efficiently pass through the opening 206a and enter the SCR integrator 200 through the light entrance plane 202. The light reflecting functions of the reflecting mirror 206 will be discussed later.
The dimension of the opening 206a is appropriately set by taking into account the entrance efficiency of the incident light from the light source unit 100 into the SCR integrator 200 and the light reflection efficiency of the reflecting mirror 206, which will be discussed later. The diameter of the opening 206a is typically about ⅓ of the longitudinal length of the reflecting mirror 206.
The reflecting mirror 206 is produced by forming an aluminum film, a silver film, or the like over the light entrance plane 202 except an area corresponding to the opening 206a. The reflecting mirror 206 may alternatively be produced by depositing a dielectric multilayer film (for example, cold mirror) or by applying an ESR film (manufactured by 3M). A flat transparent body (for example, a glass plate) with an aluminum film, a silver film, a dielectric multilayer film, or an ESR film selectively formed thereon may be disposed close to the light entrance plane 202 or may be applied on the light entrance plane 202.
Referring back to FIG. 5, the light output from the light outgoing plane 204 of the SCR integrator 200 enters the SCR wheel 300.
FIG. 7 shows the structure of the SCR wheel 300. The SCR wheel 300 has a disc-shaped filter plane 310, which is rotatable about a rotating shaft 320 by means of a motor (not shown). Borderlines of R, G, and B color filters (solid-line curves) are arranged to form a spiral of Archimedes about the center point of the rotating shaft 320 on the filter plane 310. Each of the R, G, and B color filters allows transmission of only the corresponding color light component, while reflecting the other color light components. These color filters are formed by coating the filter plane 310 with corresponding dichroic films.
The filter plane 310 of the SCR wheel 300 is disposed close to and parallel to the light outgoing plane 204 of the SCR integrator 200.
The light output from the light outgoing plane 204 of the SCR integrator 200 enters multiple color filters at a certain timing as shown by the broken line in FIG. 7. Each of the multiple color filters receiving the incident light allows transmission of only the corresponding color light component, while reflecting the other color light components.
FIG. 8 shows the light reflected by the SCR wheel 300. For example, an R light component transmission filter 310R formed on the filter plane 310 of the SCR wheel 300 receives the incident light from the light outgoing plane 204 of the SCR integrator 200, and allows transmission of only the R light component while reflecting the G light component and the B light component. The reflected G light component and B light component re-enter the SCR integrator 200 from the light outgoing plane 204.
The reflected G light component and B light component (hereafter referred to as ‘GB light components’) are repeatedly reflected inside the SCR integrator 200 and go toward the light entrance plane 202. The reflecting mirror 206 is formed on the light entrance plane 202. The GB light components reaching the light entrance plane 202 are reflected by the reflecting mirror 206, while partly passing through the opening 206a of the reflecting mirror 206.
The GB light components reflected by the reflecting mirror 206 are repeatedly reflected inside the SCR integrator 200, go toward the light outgoing plane 204, and are eventually output from the light outgoing plane 204 to re-enter the SCR wheel 300. Each of the GB light components entering a corresponding transmissible color filter on the SCR wheel 300, that is, the G light component entering a G light component transmission filter 310G or the B light component entering a B light component transmission filter 310B, passes through the SCR wheel 300 to be available as effective illumination light. Each of the GB light components entering the non-transmissible color filter, that is, the R light component transmission filter 310R, on the SCR wheel 300 is reflected again and iteratively reciprocates in the SCR integrator 200 until entering the transmissible color filter to be available as effective illumination light.
The above description regards the incident light first entering the R light component transmission filter 310R. The description is also adaptable to the incident light first entering the G light component transmission filter 310G and the incident light first entering the B light component transmission filter 310B.
This structure enables recycle of the light rays that are reflected by the SCR wheel 300 and thereby reduces the waste of light caused by the color wheel in the field sequential display, thus attaining efficient use of the light emitted from the light source unit 100. The technique of recycling the light in this manner is called the ‘SCR technique’.
The relay optical system 400 functions to focus the light passing through the SCR wheel 300 into an image on the light illumination plane of the DMD 700 at a preset imaging magnification. The relay optical system 400 has at least one focusing lens. The image formed on the light illumination plane of the DMD 700 corresponds to the illumination area. The filter plane 310 of the SCR wheel 300 is located close to the light outgoing plane 204 of the SCR integrator 200. It may thus be thought that the function of the relay optical system 400 focuses the light from the light outgoing plane 204 of the SCR integrator 200 into an image on the light illumination plane of the DMD 700 at the preset imaging magnification. The focused image has a dimension (either a dimension in the lateral direction or a dimension in the vertical direction) DB, which is expressed by Equation (2) given below:DB=DI·ks  (2)where DI denotes a dimension (either a dimension in the lateral direction or a dimension in the vertical direction) of the light outgoing plane 204 of the SCR integrator 200, and ks denotes the imaging magnification.
The imaging magnification ks is set to make the dimension (either the dimension in the lateral direction or the dimension in the vertical direction) DB of the focused image (illumination area) substantially equal to the dimension of the light illumination plane of the DMD 700. Such setting allows the light illumination plane of the DMD 700 to be efficiently illuminated. The imaging magnification ks is generally set in a range of about 1.5 times to 2.5 times.
The reflecting mirror 500 reflects the light, such that the light output from the relay optical system 400 enters the DMD 700 via the field lens 600. The reflecting mirror 500 is not an essential constituent but may be omitted.
The DMD 700 is a reflecting direction control-type light modulator, which reflects the light irradiating its light illumination plane according to image signals (image information) by means of each micromirror corresponding to each pixel and thereby makes image light, which represents an image, go out toward the projection lens 800. The image light output from the DMD 700 is projected via the field lens 600 and the projection lens 800. A resulting image corresponding to the image light is thus projected and displayed.
Due to the restrictions on the function of the DMD 700 for controlling the reflecting direction, the system optical axis 1000ax from the reflecting mirror 500 to the DMD 700 is arranged to have a predetermined gradient relative to the system optical axis 1000ax from the DMD 700 to the projection lens 800 (this is parallel to the normal line of the light illumination plane of the DMD 200). The ‘light illumination plane’ of the DMD 700 here means an area where the irradiated light is usable as image light, that is, an area with micro mirrors formed thereon. The ‘predetermined gradient’ is not essential for the characteristics of the present invention and is thus not specifically described here.
FIG. 9 schematically shows an image of the light passing through the SCR wheel 300. The light output from the light outgoing plane 204 of the SCR integrator 200 enters multiple color filters at a certain timing as shown by the broken line in FIG. 7. The light passing through the SCR wheel 300 is divided into multiple color light component areas R, G, and B as shown in FIG. 9. The borderlines of the multiple color filters shift in the radial direction with a rotation of the SCR wheel 300. With this shift, the pattern of the multiple color light component areas R, G, and B of the light passing through the SCR wheel 300 is changed, for example, from the state at a display timing T=0 shown in FIG. 9(A) to the state at a display timing T=1 shown in FIG. 9(B). The display timing represents a unit time for updating data of the displayed image.
It is thus required to supply an image signal representing a color component image, which corresponds to the color of the light irradiating each pixel of the DMD 700, to the pixel at a certain display timing. The color pattern of the irradiated light is varied, for example, with a rotation of the SCR wheel 300. The color pattern of the irradiated light may unequivocally be determined according to the position of a reference point (not shown) on the SCR wheel 300. The image processing circuit monitors the position of the reference point, specifies color pattern information of the irradiated light corresponding to the monitored position, generates the image signal based on the specified pattern information, and supplies the image signal to the DMD 700.
FIG. 10 shows problems arising in the prior art SCR projector 1000. Each drawing of FIG. 10 shows the state of dividing the image signal supplied to each pixel of the DMD 700 into multiple color light component areas at a certain display timing. The broken line in each drawing represents a color light borderline between an R light component area and a B light component area, among color light borderlines between multiple color light component areas of the light irradiating each pixel. As shown in FIG. 10(A), it is preferable that the color light borderline between the R light component area and the B light component area is coincident with a borderline between an R image area and a B image area shown by the solid line. Positional deviation of the color light borderline as shown in FIG. 10(B), blurry irradiation of the color light borderline as shown in FIG. 10(C), or distorted irradiation of the color light borderline as shown in FIG. 10(D) causes the color of the light irradiating a pixel to be different from the color of the image signal supplied to the pixel. This leads to deteriorated quality of the resulting displayed image.
In the projector based on the SCR technique, the requirement is to focus all borderlines between divisional areas of multiple color light components, which are included in the light passing through the SCR wheel, on the light illumination plane of an electro-optic device with a high accuracy, in the process of focusing the light passing through the SCR wheel into an image on the light illumination plane of the electro-optic device and thereby irradiating the light illumination area of the electro-optic device.